This invention relates to microbial insecticides. More particularly, the invention relates to a novel microbial insecticide composition and to the production and utilization thereof.
Microbial insecticides of viral, bacterial, or fungal origin offer significant advantages over conventional chemical insecticides. Microbial insect pathogens are generally nontoxic and harmless to other forms of life. In addition, microbial insecticides demonstrate a relatively high degree of specificity, and hence do not endanger beneficial insects. Moreover, a susceptible insect host is quite slow to develop resistance to microbial pathogens. Microbial insecticides may be used in relatively low dosages, may be effectively applied as dusts or sprays, and may be used in combination with chemical insecticides.
For example, the Douglas fir tussock moth nuclear polyhedrosis virus (NPV) is a microbial insect pathogen useful for controlling the tussock moth. Likewise, Bacillus thuringiensis (B.t.), a spore-forming bacterium, is well-known as a microbial insect pathogen useful against numerous leaf-chewing insects in their larval stages, including, for example, alfalfa caterpillars, tomato hornworms, tobacco hornworms, cabbage loopers, cabbage web worms, army worms, gypsy moths, walnut caterpillars, diamondback moths, cosmopolitan green beetles, European corn borers, and other members of the order Lepidoptera.
Unfortunately, the effectiveness and usefulness in the field of many microbial insect pathogens as insecticides are severely limited by their extreme sensitivity to sunlight. It is known, for example, that one of the problems encountered when using B.t. as an insecticide is its short period of effectiveness in the field, which is due, in part, to sunlight-induced inactivation of the microorganism. It is also known that nonionizing radiation having a high photon energy (e.g. ultraviolet rays) exerts an inactivating effect on B.t.. See, "Photoprotection Against Inactivation of Bacillus thuringiensis Spores by Ultraviolet Rays," Aloysius Krieg, Journal of Invertebrate Pathology, Vol. 25, pp. 267-268 (1975). In particular, it is known that ultraviolet (UV) rays with a wavelength of 253.7 nm induce a marked, extraordinary inactivation of B.t. spores, so that they are unable to germinate and grow out. A dosage of 18 m W sec/cm.sup.2 of such 253.7 nm wavelength radiation will inactivate 99.9% of the B.t. spores. However, since UV radiation of wavelengths shorter than about 285 nm do not reach the earth's surface, such inactivation at 253.7 nm is of little practical concern in the field.
We have determined that the half life of B.t. subjected to sunlight is approximately six minutes. Likewise, it has been determined by others that the half lives of certain occluded viruses subjected to sunlight is one-half to one hour. Thus, the effectiveness of a typical spray application of such microbial insecticides is rapidly lost in the field.
Since nucleic acids show a maximum of extinction near a wavelength of 260 nm, it has been suggested by others that the UV induced death of B.t. at 253.7 nm, and of certain occluded viruses at comparable wavelenths, may be caused by a photoreaction of the genetic material, especially DNA. Thus, it has been suggested, ibid., at p. 267, that B.t. spores could be protected from inactivation by such UV radiation (253.7 nm) by physically mixing the B.t. spores with DNA, or a comparable nucleic acid which would absorb the UV rays. Such a comparable nucleic acid would be RNA, Ribonucleic Acid, which has a maximum of extinction near 260 nm. However, this technique proved to be ineffective. Furthermore, as noted above, since wavelengths shorter than about 285 nm do not reach the earth's surface, the usefulness of DNA or RNA as a protectant against sunlight-induced (i.e. at wavelengths greater than about 285 nm) inactivation is unproven.